Test-retest reliability, agreement, and minimal detectable change in the 6-minute walk test in patients with intermittent claudication

Objective: Standardized walk tests are important for objective assessment of walking distance in patients with intermittent claudication (IC). The 6-minute walk test (6MWT) has been suggested to correlate more closely than testing on a treadmill with everyday ambulatory function, but its measurement properties have hardly been studied in IC. The aim of this study was to determine the test-retest reliability, agreement, standard error of measurement (SEM), and minimal detectable change of the 6MWT in patients with IC. Methods: This reliability and agreement study recruited 102 patients with stable IC (mean age, 72 6 +/- 7.4 years; 43 women) from the vascular surgery outpatient clinic at Sahlgrenska University Hospital in Sweden. The patients performed the 6MWT twice, with at least 30 minutes of rest between tests. To determine test-retest reliability, the intraclass correlation coefficient was calculated. Bland-Altman plots were used to measure agreement. Results: The mean walking distance in both test and retest was 397.8 m (standard deviation, 81.2 m; N = 100), and the individual walking distance varied from 175 to 600 m. Excellent test-retest reliability for the 6MWT (intraclass correlation coefficient, 0.95; 95% confidence interval, 0.9-0.97) was observed. The SEM was 16.6 m (95% confidence interval, 14.6-19.3), the SEM percentage was 4.2%, and the minimal detectable change was 46 m. Five observations (5%) were positioned outside the limits of agreement; there was a small proportional bias, and the scatter of values for differences decreased as the average values increased. Conclusions: The excellent test-retest reliability implies that it is sufficient for a patient with IC to perform the 6MWT once, at every test occasion. For the individual, an improvement or deterioration in maximum walking distance of >46 m after an intervention would be required to be 95% confident that the change is significant. Being a simple and clinically useful test, the 6MWT can be widely used to evaluate the effects of different interventions in patients with IC.